Solid state fluid level sensor

ABSTRACT

A sensor system for sensing liquid level in a bilge, for use in automatic bilge pump actuation. First and second field effect sensors are potted or sealed within a container or the bilge wall and are aligned in a vertical array and each comprise a substantially planar pattern of “electrodes” or conductive traces disposed on a printed circuit board (PCB) along with integrated circuits used to create a loop or arc-shaped electric field. As bilge liquid rises to the proximity or level of the field effect sensors, a change in the arc-shaped electric field is sensed and, in response, a bilge pump is automatically actuated to pump liquid out of the bilge. Optionally, the pump control can be programmed by use of a microprocessor to permit control of on-off timing and prevent undesirable effects of “sloshing.”

The present application is a divisional of U.S. patent application Ser.No. 12/976,143 filed on Dec. 22, 2010, abandoned, which is a divisionalof U.S. patent application Ser. No. 12/117,789, filed May 9, 2008, nowU.S. Pat. No. 8,024,967, which is a divisional of U.S. Pat. No.7,373,817, issued May 20, 2008, the entire disclosures of which arehereby incorporated by reference in their entirety as if fully set forthherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sensors for detecting the presence of afluid, automatic systems for actuating pumps in response to detecting afluid level and sensors mounted in the bottom of a boat bilge tankactivating a bilge pump when the bilge fluid level reaches a presetdistance above the bottom of the bilge tank.

2. Discussion of the Prior Art

In the past, bilge pumps have been activated manually or by mechanicalfloat type switches with mercury or point contacts to complete anelectrical circuit activating a pump. Pressure switches have also beenused. These prior art switches worked adequately when initiallyinstalled. Over time, however, bilge debris and other sources ofcontamination often prevented the mechanical components from moving asintended, causing switch failure. In addition, prior art bilge pumpactivation switches typically wore out several times during the life ofa boat and, being located in a boat's nether regions, were difficult toaccess for repair and replacement.

Many fluid level or fluid proximity detectors of the prior art employedelectrical switches actuated when a conductor, such as a body of water,moved into close proximity to the detector or sensor. U.S. Pat. Nos.3,586,859; 3,665,300; 4,800,755 and 4,875,497 disclose such detectors.U.S. Pat. No. 5,017,909, discloses a proximity detector used as a liquidlevel detector for receptacles in vehicles.

Other applications for liquid level detectors included bilge-pumpingsystems for ships. A bilge pumping system must be activated before theaccumulated water reaches an excessive level. Prior art mechanisms fordetection of an excessive bilge water level employed mechanicalfloatation systems, causing a switch to be actuated whenever the waterreached such an undesired level. Bilge fluid or water eventually rendersmechanical level sensing systems inoperative in part because bilge fluidcan contain many forms of corrosive waste. Replacing failed parts of abilge level sensing system can be very expensive and troublesome, sincea skilled technician must enter the bilge to perform the work.

Many electronic proximity detection systems have been proposed insearching for a solution to this messy, expensive problem. By way ofexample, Smith et al (U.S. Pat. No. 4,881,873) discloses capacitivelevel sensor for a bilge pump including a sensor plate 40 positioned ina bilge at a position selected for pump actuation. The bilge water issensed as a dielectric, in a manner of speaking, and so the sensor issusceptible to false alarms or missed detections once the contaminationaccompanying bilge inflow has accumulated in the bilge and contaminatedthe area around the sensor, and sloshing bilge water is likely to causethe bilge control to actuate when the bilge level does not requirepumping.

Gibb (U.S. Pat. No. 5,287,086) also discloses a capacitive level sensorfor a bilge pump including a capacitive sensor plate 79 positioned in abilge at a position selected for pump actuation. The sensor is containedwithin a sealed housing 32 to keep bilge water away from the sensor andother circuitry. Here again, bilge water is sensed as a dielectric, in amanner of speaking, and so the sensor is susceptible to false alarms ormissed detections once the contamination accompanying bilge inflow hasaccumulated in the bilge and contaminated the area around the sensor,and sloshing bilge water is likely to cause the bilge control to actuatewhen the bilge level does not require pumping.

Santiago (U.S. Pat. No. 4,766,329) discloses a solid-state two levelsensor for a bilge pump including a high water level probe and a lowwater level probe, both positioned in a bilge at positions selected forpump actuation. The probes are in contact with the bilge water, and sothe probe sensors are susceptible to false alarms or missed detectionsonce the contamination accompanying bilge inflow has accumulated in thebilge and contaminated the probes.

Farr (U.S. Pat. No. 5,238,369) discloses a system for liquid levelcontrol including upper and lower capacitive level sensors 10, 18 havingpositions selected for pump actuation. This reference is silent on theneed to keep bilge water away from the sensors, but the bilge water issensed as a dielectric, in a manner of speaking, and so the sensor issusceptible to false alarms or missed detections once the contaminationaccompanying bilge inflow has accumulated in the bilge and contaminatedthe area around the sensors.

The applicant has licensed a Field Effect sensor patent to Caldwell(U.S. Pat. No. 5,594,222) on a “touch sensor” used to detect whether auser presses a virtual button; this sensor is referred to as a “touchsensor.” While the patent discloses the electromagnetic properties ofField Effect “touch” sensing, it is silent on how such technology mightbe employed in a sensor system for detecting a fluid/air interface orfor automated bilge pump actuation.

There is a need, therefore, for a system for sensing liquid level andliquid level control that overcomes the problems with prior art sensorsand systems, permitting installation of a reliable, inexpensive fluidlevel sensing system which is unlikely to require maintenance orcleaning in the bilge. It would be highly desirable to have a new andimproved proximity detection system which is highly reliable andrelatively inexpensive to manufacture. Such a proximity detection systemshould be highly sensitive and possess a wide range of applications.

SUMMARY OF THE INVENTION

The fluid level control and sensor system of the present inventioncomprises a fluid tight housing or container and a circuit board withelectrodes and interconnect patterns assembled with components to createan electric field having arc shaped patterns and sense changes using thefield effect principle.

The housing or container holds first and second electrode patterns in avertical orientation and has ribs on the housing's sides to allow debrisfound in a bilge to slough off. By careful selection of materials, thecontainer can resist biological attack (e.g., fungus or algae) andprevent fouling from other materials that might stick to the containerotherwise. In the vertical position, gravity also helps to allow theanticipated contamination to slough off.

The field effect is described elsewhere in U.S. Pat. No. 5,594,222 andothers assigned to TouchSensor, LLC, which are typically used in largeappliance applications for operator input. A similar principle isadapted to detect liquids in close proximity to the sensor, even whenisolated from the liquid by a physical barrier, such as a tank wall ormolded container. This technique rejects common mode contamination tothe sensor and, through proper tuning of the device, allows the presenceor absence of a liquid to be detected. Since this is a solid statedevice in close proximity to the detector and it is also low impedance,it is also very tolerant of electrical noise in the marine environment.

The electrode design can have geometries ranging from parallel plates toconcentric rings of various sizes and geometric shapes. The design ofthe electrodes is determined by the materials of construction,thickness, composition of the liquid and other considerations.

Since the fluid level control and sensor system is submerged when activeand passing current, an internal current switching device (e.g. a FieldEffect Transistor (FET)) adapted to pass twenty amps is easily cooled.

Electronics to support the sensor optionally include components allowingcontrol of devices demanding twenty amps of current without the additionof a separate relay. Through current scalping and other techniques, thebilge pump control system of the present invention operates through twowires or can have a separate third wire to provide power. The fluidlevel control and sensor system can be implemented with or without amicroprocessor.

The bilge pump assembly consists of a housing, circuit board, componentsand wiring harness, in operation, the bilge pump sensor is installedin-line between the pump and a power supply. The circuit draws its ownpower from the power mains supply without activating the pump. Withoutbilge liquid present near the sensor electrodes, a sensing IC is at afirst state, “off.” With a liquid present near the sensor electrodes,the IC changes state to “on” and the circuitry allows connection ofpower to the bilge pump. The pump is activated until the liquid levelgoes below the sensor electrodes. The sensing IC changes state to “off”and the power to the pump is interrupted, causing the pump to stop. Thesequence is repeated whenever liquid comes in proximity to the sensor.An optional microprocessor allows control of on-off timing and othertime management operations provide a stable pump operation without rapidchanges from the “on” to the “off” state (e.g., due to an instabilityreferred to as “sloshing”). Sloshing may vary in amplitude depending onthe length of craft and bilge tank and the rocking motion of the craft.

The bilge pump controller of the present invention includes a fieldeffect sensor comprising an active, low impedance sensor on a dielectricsubstrate. The sensor has a first conductive electrode pad and a secondconductive electrode which substantially surrounds the first electrodein a spaced apart relationship. The first electrode pad has a closed,continuous geometric shape and both electrodes are attached to the samesurface of the substrate. An active electrical component is placed inclose proximity to the electrodes.

The sensor is used to replace conventional switches and is activatedwhen bilge fluid or water contacts or comes into close proximity withthe substrate. The sensor is used to turn an electric pump motor on oroff. The field effect sensor design operates properly with liquidspresent on the substrate and in the presence of static electricity, andis well-suited for use in an environment where water, grease and otherliquids are common, such as boat bilges or other sea-going applications.

Electrodes are attached to the back surface of a substrate, opposite thefront or “wet” surface, thereby preventing contact between theelectrodes and the controlled fluid (e.g., bilge water). Since thesensor electrodes are not located on the wet surface of the substrate,the sensor is not damaged by scratching, cleaning solvents or any othercontaminants which contact the substrate.

The cost and complexity of the sensor is reduced since a relay or switchis not required.

In the preferred form, an oscillator is electrically connected to theinner and outer electrodes through gain tuning resistors and delivers asquare-wave like signal having a very steep slope on the trailing edge.The oscillator signal creates an arc shaped transverse electric fieldbetween the outer electrode and the center electrode. The electric fieldpath is arc-shaped and extends through the substrate and past the frontsurface, projecting transversely to the plane of the substrate. Theinner and outer electrode signals are applied as common mode signals tothe inputs of a differential sensing circuit and when the difference inresponse between the inner and outer electrodes is great enough, thesensing circuit changes state (e.g., from high to low). The sensingcircuit state is altered when the substrate is touched by the controlledfluid.

In the preferred form, an active electrical component preferablyconfigured as a surface mount application specific integrated circuit(ASIC), is located at each sensor. Preferably, the ASIC is connected tothe center pad electrode and to the outer electrode of each sensor. TheASIC acts to amplify and buffer the detection signal at the sensor,thereby reducing the difference in signal level between individualsensors due to different lead lengths and lead routing paths. Aplurality of sensors may be arranged on the substrate.

The above and still further features and advantages of the presentinvention will become apparent upon consideration of the followingdetailed description of a specific embodiment thereof, particularly whentaken in conjunction with the accompanying drawings, wherein likereference numerals in the various figures are utilized to designate likecomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, in elevation, of a bilge fluid level control andsensor system, in accordance with the present invention.

FIG. 2 is a partial cross section, in elevation, of the fluid levelcontrol and sensor system of FIG. 1, in accordance with the presentinvention.

FIG. 3 is a back side view, in elevation, of the primary, back or drysurface of a printed wiring board assembly for the fluid level controland sensor system of FIGS. 1 and 2, in accordance with the presentinvention.

FIG. 4 is a schematic diagram of the printed wiring board assembly forthe fluid level control and sensor system of FIGS. 1, 2 and 3, inaccordance with the present invention.

FIG. 5 is a diagram illustrating a field effect fluid level sensorsystem showing the arc-shaped field passing through the fluid, inaccordance with the present invention.

FIG. 6 is a diagram drawn to scale and illustrating the component sidelayout of conductive traces on a printed circuit board including abalanced pad sensor electrode pattern.

FIG. 7 is an edge view, in elevation, illustrating the conductive traceson the printed circuit board of FIG. 6 including the balanced pad sensorelectrode pattern, as seen from the side.

FIG. 8 is an edge view, in elevation, of an alternative two-sidedembodiment illustrating the conductive traces on a printed circuit boardincluding the balanced pad sensor electrode pattern, as seen from theside.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the exemplary embodiment illustrated in FIGS. 1-5, a bilgeor fluid containment vessel 10 is bounded by a fluid tight wall orsurface 18 and at times contains bilge water or some other fluid 12. Thefluid level in the bilge rises and falls and the fluid level ismeasurable over a selected dimension such as that shown by the verticalscale in FIG. 1. As the fluid level rises or falls, a fluid/airinterface 16 can be sighted or measured along the fluid level scale 14.In the typical marine application, bilge 10 contains fluid 12 such aswaste water or seawater that leaks through the hull or deck, and whenthe bilge fluid level is excessively high (e.g., at a selected upper ortrigger level 16H), fluid 12 must be pumped out, usually with anelectrically powered pump (not shown) that is selectively energized whenthe excessively high fluid level 16H is detected. The fluid level issensed while pumping progresses and the pump is turned off when thelevel of fluid 12 is low enough (e.g., at a selected lower or turn-offlevel 16L).

As shown in FIGS. 1, 2 and 3, fluid level control and sensor system 20of the present invention comprises a fluid tight housing or container 24and a printed wiring board assembly 22 with electrodes 50, 52, 60 and 62and interconnect patterns assembled with components to create anelectric field having arc shaped patterns and sense changes using the“field effect” principle.

Referring to FIGS. 4 and 5, the field effect is generated and changes inthe field are sensed using patterns of conductive traces or electrodes50, 52 which create arc shaped electric fields 70 projectingtransversely and passing through substrate 30. When a rising fluid/airinterface 16 is sensed, the arc shaped electric fields 70 pass throughthe fluid 12 rather than the air above the fluid/air interface. Theoperation of the sensor system 20 will be described in greater detailbelow.

In the embodiment of FIG. 1, printed wiring board assembly 22 isdisposed within a cavity 26 of housing or container 24 and, duringassembly, is sealed inside to provide a fluid proof barrier ofsubstantially uniform thickness. Printed wiring board or circuit board30 is a dielectric planar substrate of substantially uniform thicknesshaving a back side (shown in FIGS. 2 and 3) carrying the components andopposite a front side. The sensor system 20 is connected to a mainssupply and an electric pump (not shown) via a two cable wire assembly 32preferably comprising at least first and second sixteen gauge (16AWG)wire segments.

The electrical components in the sensor embodiment illustrated in FIGS.2, 3 and 4 include a pre-programmed integrated circuit (IC) eight bitmicro-controller also identified as U4, which is connected to andresponsive to first and second touch switch ICs, 36, 37 also identifiedas U2 and U3, respectively. A sensor system power supply includesvoltage regulator 38, also identified as U1, and the mains supplyvoltage is also controlled by a pair of diodes including seriesconnected diode 46 and shunt connected zener diode 44 providing power toa voltage regulator 38.

In the illustrated embodiment, the bilge water pump actuation and supplycomes from a large switching N channel Field Effect Transistor (NFET)42, the mains supply voltage (e.g., 12 VDC) is connected throughconnector J1 to the drain and the current, when switched on to actuatethe pump, passes through the source and through connector J2 to thepump.

As best seen in FIG. 3, a first sensor electrode pattern 49 includes asubstantially rectangular upper center electrode pad 50 including aplurality of vertical conductive traces separated by approximately equalwidth sections of non-conductive PCB surface, where each conductivetrace is connected at its upper and lower ends by a surroundingconductive trace, all connected to resistor R5. Upper center electrodepad 50 is not quite completely encircled by upper outer electrode 52which is connected to upper touch sensor ASIC 36. Both inner pad 50 andouter electrode 52 are at least partly encircled by a perimeter of solidconductive trace material to provide a ground ring (not shown). Theground ring can be configured to influence sensitivity anddirectionality of outer electrode 52; if the ground ring is situated tooclosely to outer electrode 52, however, the sensor's differential modeis lost and the sensor behaves in a single ended fashion.

A second sensor electrode pattern 59 is disposed in a vertically alignedorientation below the first sensor electrode pattern 49 and includes asubstantially rectangular lower center electrode pad 60 including aplurality of vertical conductive traces separated by approximately equalwidth sections of non-conductive PCB surface, where each conductivetrace is connected at its upper and lower ends by a surroundingconductive trace, all connected to resistor R7. Lower center electrodepad 60 is not quite completely encircled by lower outer electrode 62which is connected to lower touch sensor ASIC 37. Both pad 60 and outerelectrode 62 are also at least partially encircled by a second perimeterof solid conductive trace material to provide a second ground ring (notshown).

In accordance with the present invention, the field effect principle isadapted to detect liquid 12 when in close proximity to sensor 20, evenwhen isolated from the liquid by a physical barrier such as a tank wallor molded container (e.g., 24). This technique rejects common modecontamination to the sensor and, through proper tuning of the device,allows the presence or absence of liquid or fluid (e.g., bilge water 12)to be detected. Since the touch sensors ASICs 36, 37 are solid statedevices in close proximity to the electrode pads and also of lowimpedance, they are also very tolerant of electrical noise in the marineenvironment.

The electrode design can have geometries ranging from parallel plates toconcentric rings of various sizes and geometric shapes. The design ofthe electrodes (e.g., 50, 52, 60 and 62) is determined by the materialsof construction, thickness, composition of liquid 12 and otherconsiderations, as will be appreciated by those with skill is the art.

In the illustrated embodiment, the electronics supporting the sensorinclude components allowing control of devices (e.g., a pump) demanding20 amps of current without the addition of a separate relay. Since fluidlevel control and sensor system 20 is submerged when active and passingcurrent, heat sinking to the bilge fluid 12 allows an internal currentswitching device (e.g. NFET 42) and a twenty amp supply circuit to beeasily cooled.

Through current scalping and other techniques, the fluid level controland sensor system 20 operates through two wires or can have a separatethird wire (not shown) to provide power. Current scalping or scavengingutilizes the error output of Voltage regulator 38 as an input tomicroprocessor 34 which is programmed to turn off FET switch 42 for ashort interval during which shunt storage capacitor C1 charges back to12 volts. This current scalping method permits an “on-time” for FETswitch 42 of at least 97%.

Fluid level control and sensor system 20 can be implemented with orwithout a microprocessor (not shown).

Housing or container 24 supports and protects first and second electrodepatterns 49 and 59, in a vertical orientation and optionally has one ormore external ribs transversely projecting vertically aligned elongatedfeatures 27 on the housing's sides to allow bilge debris to slough offor away from housing 24 as the fluid level falls during pumping. Housing24 is preferably made of inert materials such as ABS, polypropylene orepoxy to resist biological attack (e.g., fungus or algae) and preventfouling from other substances that might otherwise tend to stick to thehousing or container sidewalls. In the vertical orientation or positionshown in FIG. 1, gravity also helps to allow the anticipatedcontamination to slough off.

In operation, the bilge pump sensor 20 is installed in-line between thepump and a power mains supply using wire assembly 32. The circuit drawsits own power from the power mains supply through connector J1 withoutactivating the pump. Without bilge liquid present near the sensorelectrodes, each of the sensing ICs 36, 37 is at a first state, “off.”With liquid 12 present near the sensor electrodes, the sensing ICs 36,37 change state to a second state, “on”, and the circuitry connectspower through FET 42 to the bilge pump. The pump remains activated untilthe sensed liquid level 16 goes below the sensor electrodes 49 and 59.The sensing ICs 36, 37 then change state to “off” and the power to thepump is interrupted, causing the pump to stop. This sequence is repeatedwhenever the fluid surface 16 comes into proximity with the sensorsystem 20. An optional microprocessor allows control of on-off timingand other time management operations to provide a stable pump operationwithout rapid changes from the “on” to the “off” state (e.g., due to aninstability referred to as “sloshing”). Sloshing may vary in amplitudedepending on the length of craft and bilge tank and the rocking motionof the craft.

The bilge pump controller or sensor system 20 of the present inventionincludes at least one field effect sensor comprising an active, lowimpedance sensor attached to only one side of dielectric substrate 30and is used to replace conventional switches. The field effect sensordesign operates properly with liquids present on the substrate and inthe presence of static electricity, and is well-suited for use in amarine environment where water, grease and other liquids are common,such as boat bilges or other sea-going applications.

As shown in FIG. 5, the sensor's printed wiring board assembly 22 may bemolded or fabricated into the bilge sidewall 18 rather than beingseparately encapsulated in housing 24, with the front or “wet” side ofthe PC board 30 facing the interior of the bilge 10.

Preferably, sensor electrode patterns 49, and 59 are attached to theback surface of PCB substrate 30. The back surface of the substrate isopposite the front or “wet” surface, thereby preventing contact of theelectrodes by the controlled fluid (e.g., bilge water). Since the sensorelectrodes are not located on the front surface of the substrate, thesensor is not damaged by scratching, cleaning solvents or any othercontaminants which contact the front surface of the substrate.Furthermore, the cost and complexity of the sensor is reduced since aswitch is not required.

Preferably, an active electrical component, such as a surface ASIC(e.g., 36) is located at each sensor and connected between the centerelectrode (e.g., 50) and the outer electrode (e.g., 52) of each sensor.The ASIC acts to amplify and buffer the detection signal at the sensor,thereby reducing the difference in signal level between individualsensors due to different lead lengths and lead routing paths.

The Integrated circuit connected to the field effect sensor electrodesis an active device and, in the illustrated embodiment, is preferablyconfigured as ASIC operating in the manner described in U.S. Pat. No.6,320,282, to Caldwell, the entire disclosure of which is incorporatedherein by reference. As described above, a simple field effect cell hastwo electrodes (e.g., 50, 52), an ASIC (e.g., 36) and two gain tuningresistors (e.g., R5 and R6). The pin-out for the TS-100 ASIC of theinvention is similar to that illustrated in FIG. 4 of the '282 patent,but the pin-outs vary slightly. The TS-100 ASIC is available from TouchSensor, LLC. Specifically, for the TS-100 ASIC shown in thisapplication, the input power (Vdd) connection is on pin 1, the groundconnection is on Pin 2, the sensor signal output connection is on pin 3,the outer electrode resistor (e.g., R6) is connected to pin 4, the“oscillator out” connection is at pin 5 and the inner pad electroderesistor (e.g., R5) is connected to pin 6. Optionally, an ASIC can beconfigured to eliminate the need for gain tuning resistors R5 and R6 bymaking the gain tuning adjustments internal to the ASIC.

The sensitivity of the field effect sensor or cell is adjusted byadjusting the values of gain tuning resistors R5 and R6. The sensor ofthe present invention can be adapted for use in a variety ofapplications and the gain resistors R5 and R6 can be changed to cause adesired voltage response. The sensor of the present invention is likeother sensors in that the sensor's response to measured stimulus must betuned or calibrated to avoid saturation (i.e., from gain/sensitivity settoo high) and to avoid missed detections (i.e., from gain/sensitivityset too low). For most applications, a gain tuning resistor value whichyields a sensor response in a linear region is preferred. The tuning orcalibration method typically places the sensor assembly in the intendedsensing environment and the circuit test points at the inputs to thedecision circuit (e.g., points 90 and 91 as seen in FIG. 4 of Caldwell's'282 patent) are monitored as a function of resistance. The resistancevalue of the gain tuning resistors R5 and R6 are adjusted to provide anoutput in the mid-range of the sensor's linear response.

While other electrode patterns are suitable for this bilge pump controlapplication, the illustrated electrode patterns 49 and 59, as shown in3, are each balanced. “Balanced” as used here, means that the conductivetrace area of the inner electrode (e.g., center pad 50) is equal (or asequal as possible within PCB manufacturing tolerances) to the area ofits corresponding outer electrode ring (e.g., outer electrode 52).

The applicants have discovered that the illustrated balanced padelectrode design provides improved noise or electromagnetic interference(EMI) immunity and works exceptionally well for sensing the presence ofa fluid such as water.

The EMI immunity appears to stem from a common mode rejection ofspurious noise or interference signals. This “common mode” rejection isattributable to the equal area of the center pad and the outer ringelectrode, which appear to be affected by spurious noise or interferencesignals substantially equally, and so when one electrode's signal issubtracted from the other electrode's signal, the commonnoise/interference signals cancel one another.

In the embodiment illustrated in FIG. 3, each balanced center pad issubstantially rectangular having a horizontal extent of approximatelytwelve millimeters (mm) and a vertical extent of nine mm. As can be seenfrom FIG. 3, each center electrode pad (e.g., 50 and 60) include aplurality of vertical conductive traces (each approx. one mm in width)separated by approximately equal width sections of non-conductive PCBsurface, where each conductive trace is connected at its upper and lowerends by a surrounding conductive trace material, all connected to aresistor (e.g., R5 for upper pad 50). Each center electrode pad (e.g.,50) is not quite completely encircled by upper outer electrode 52 whichis approximately 1.5 mm in width and is connected to an ASIC (e.g., 36).

An alternative electrode pattern embodiment is illustrated in FIGS. 6and 7; FIG. 6 is a diagram drawn to scale and illustrating the componentside layout of conductive traces on a printed circuit board including abalanced pad sensor electrode pattern 69, and FIG. 7 is an edge view, inelevation, illustrating the conductive traces of balanced pad sensorelectrode pattern 69, as seen from the side of the PCB (e.g., such asPCB 30). As noted above, a balanced pad or electrode pattern has aconductive trace area for the inner electrode (e.g., center pad 70) thatis equal (or as equal as possible within PCB manufacturing tolerances)to the area of its corresponding outer electrode ring (e.g., outerelectrode 72). Both pad 70 and outer electrode 72 are also optionallyencircled by a perimeter of solid conductive trace material to provide aground ring 76.

The optional ground ring 76 is useful for reducing the effects of theboat or bilge material on the operation of the sensor 20. When a groundring 76 is included, sensor 20 may be installed in either a conductive(e.g. aluminum or steel) bilge or a dielectric (e.g., fiberglass) bilgewith negligible effects on sensor performance.

An alternative electrode pattern embodiment is illustrated in FIG. 8, adiagram illustrating an edge view of a two-sided layout of conductivetraces on a printed circuit board including a balanced pad sensorelectrode pattern 79, as seen from the side of the PCB (e.g., such asPCB 30). As above, a balanced pad or electrode pattern has a conductivetrace area for the inner electrode (e.g., center pad 80) carried on oneside of the PCB that is equal (or as equal as possible within PCBmanufacturing tolerances) to the area of its corresponding outerelectrode ring (e.g., outer electrode 82) on the opposite side of PCB30. Both pad 80 and outer electrode 82 are at least partially encircledby a perimeter of solid conductive trace material to provide a groundring 86 which is spaced apart from outer electrode 82.

An alternative embodiment with microprocessor control can be programmedto provide delays for pump turn-on and turn-off. The micro-controllerhas been programmed to read the lower level pad 60 and the upper levelpad 50. Both pads must be activated for at least 256 consecutive readsbefore the bilge pump is turned on. The interval between reads is 0.0011seconds, resulting in 0.286 seconds of total time of continuous readswith both pads active before turning on the pump. If at any time, eithersensor is not active, the read is canceled and no pump enabling actionis taken. During periods when the pump is not active, the controllerputs itself to sleep for 2.5 seconds and then wakes up and reads bothupper and lower pads 50, 60. Thus, the time between water level readingsis about 2.78 seconds. This pre-programmed algorithm would have a highprobability of not turning on the pump if the water is merely sloshingaround in bilge 10, but gives fairly quick response. This pre-programmedalgorithm is preferably stored in a memory and operates in a mannersimilar to the principals of fuzzy logic.

Other methods may be used to overcome “sloshing” Various electronicmethods include using simple timing circuits like “one shots”, “pulseswitching and timing circuits”, and “low pass filtering”. None of thesewould be as economical as using the single chip micro-computerincorporating the entire control function in simple circuit using amicro, however.

A mechanical anti-sloshing control method includes configuring a plasticchamber or shield around the sensing area and allowing water to slowlyflow in and out of the chamber and sensing area thereby preventing waterlevel from changing very rapidly in the area of the sensor. This dampedflow method would not prevent sensors from switching on/off rapidly whenthe water level was at the very edge of activation, where slightmovements of water or electrical noise would cause sensor oscillation.The mechanical anti-slosh chamber could be coupled with a simpleelectronic filter circuit such as a low pass and comparator, but thiswould require more plastic than the illustrated design. The mechanicalmethod of slosh proofing could also be used in conjunction with ourcurrent micro-controller design. The anti-sloshing chamber may createproblems if the water entry points were to get clogged due to dirtand/or debris.

The illustrated micro controller solution results in the simplest,smallest, cheapest, lowest energy consumption result with maximumflexibility and reliability, and so is preferable to non-micro solutionsor mechanical methods.

Having described preferred embodiments of a new and improved method, itis believed that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims.

The invention claimed is:
 1. A bilge pump, comprising: a first fieldeffect sensor, within a sealed housing within a bilge, the first fieldeffect sensor configured to generate a first electromagnetic field anddetect a presence of fluid proximate to the first field effect sensor bydetecting variations in the first electromagnetic field; a second fieldeffect sensor, vertically aligned below the first field effect sensorwith respect to the bilge, the second field effect sensor configured togenerate a second electromagnetic field and detect a presence of thefluid proximate to the second field effect sensor by detectingvariations in the second electromagnetic field; and an actuatorconfigured to operate an electric pump in response to the first fieldeffect sensor detecting the presence of the fluid proximate to the firstfield effect sensor and stop operation of the electric pump in responseto the second field effect sensor detecting the presence of the fluidproximate to the second field effect sensor.
 2. The bilge pump of claim1, wherein the first field effect sensor comprises a first electrodehaving a first trace area and a second electrode at least partiallysurrounding the first electrode, the second electrode having a secondtrace area substantially equal to the first trace area.
 3. The bilgepump of claim 1, wherein the second field effect sensor comprises athird electrode having a third trace area and a fourth electrode atleast partially surrounding the third electrode, the fourth electrodehaving a fourth trace area substantially equal to the third trace area.4. The electric pump of claim 1, wherein the actuator is configured tooperate the electric pump in response to the first field effect sensordetecting the presence of fluid proximate to the first field effectsensor for a predetermined period of time.
 5. A bilge pump, comprising:a first field effect sensor within a sealed housing within a bilge, thefirst field effect sensor comprising: a first electrode having a firsttrace area; and a second electrode at least partially surrounding thefirst electrode, the second electrode having a second trace areasubstantially equal to the first trace area, wherein the first fieldeffect sensor is configured to generate a first electromagnetic fieldand detect a presence of fluid proximate to the first field effectsensor by detecting variations in the first electromagnetic field; andan actuator configured to operate an electric pump in response to thefirst field effect sensor detecting the presence of the fluid proximateto the first field effect sensor.
 6. The bilge pump of claim 5, furthercomprising a second field effect sensor vertically aligned below thefirst field effect sensor with respect to the bilge.
 7. The bilge pumpof claim 6, wherein the second field effect sensor is configured to:generate a second electromagnetic field; and detect a presence of thefluid proximate to the second field effect sensor by detectingvariations in the second electromagnetic field.
 8. The bilge pump ofclaim 7, wherein the actuator is configured to stop operation of theelectric pump in response to the second field effect sensor detectingthe presence of the fluid proximate to the second field effect sensor.9. The bilge pump of claim 6, wherein the second field effect sensorcomprises a third electrode having a third trace area and a fourthelectrode at least partially surrounding the third electrode, the fourthelectrode having a fourth trace area substantially equal to the thirdtrace area.
 10. The electric pump of claim 5, wherein the actuator isconfigured to operate the electric pump in response to the first fieldeffect sensor detecting the presence of fluid proximate to the firstfield effect sensor for a predetermined period of time.
 11. The bilgepump of claim 5, further comprising a second field effect sensor havinga third electrode having a third trace area and a fourth electrode atleast partially surrounding the third electrode, the fourth electrodehaving a fourth trace area substantially equal to the third trace area.12. A method of pumping fluid from a bilge, the method comprising:providing a first field effect sensor within a sealed housing within thebilge; generating, by the first field effect sensor, a firstelectromagnetic field; detecting a presence of the fluid proximate tothe first field effect sensor by detecting variations in the firstelectromagnetic field; providing a second field effect sensor,vertically aligned below the first field effect sensor with respect tothe bilge; generating, by the second field effect sensor, a secondelectromagnetic field; detecting a presence of the fluid proximate tothe second field effect sensor by detecting variations in the secondelectromagnetic field; operating an electric pump in response to thefirst field effect sensor detecting the presence of the fluid proximateto the first field effect sensor; and stopping operation of the electricpump in response to the second field effect sensor detecting thepresence of the fluid proximate to the second field effect sensor. 13.The method of claim 12, wherein the first field effect sensor comprisesa first electrode having a first trace area and a second electrode atleast partially surrounding the first electrode, the second electrodehaving a second trace area substantially equal to the first trace area.14. The method of claim 13, wherein the second field effect sensorcomprises a third electrode having a third trace area and a fourthelectrode at least partially surrounding the third electrode, the fourthelectrode having a fourth trace area substantially equal to the thirdtrace area.
 15. The method of claim 12, wherein operating the electricpump in response to the first field effect sensor detecting the presenceof the fluid proximate to the first field effect sensor comprisesoperating the electric pump in response to the first field effect sensordetecting the presence of the fluid proximate to the first field effectsensor for a predetermined period of time.
 16. A method of pumping fluidfrom a bilge, the method comprising: providing a first field effectsensor within a sealed housing within the bilge, the first field effectsensor comprising a first electrode having a first trace area and asecond electrode at least partially surrounding the first electrode, thesecond electrode having a second trace area substantially equal to thefirst trace area; generating, by the first field effect sensor, a firstelectromagnetic field; detecting a presence of the fluid proximate tothe first field effect sensor by detecting variations in the firstelectromagnetic field; and operating an electric pump in response to thefirst field effect sensor detecting the presence of the fluid proximateto the first field effect sensor.
 17. The method of claim 16, furthercomprising: providing a second field effect sensor vertically alignedbelow the first field effect sensor with respect to the bilge.
 18. Themethod of claim 17, further comprising: generating, by the second fieldeffect sensor, a second electromagnetic field; and detecting a presenceof the fluid proximate to the second field effect sensor by detectingvariations in the second electromagnetic field.
 19. The method of claim18, further comprising: stopping operation of the electric pump inresponse to the second field effect sensor detecting the presence of thefluid proximate to to the second field effect sensor.
 20. The method ofclaim 17, wherein the second field effect sensor comprises a thirdelectrode having a third trace area and a fourth electrode at leastpartially surrounding the third electrode, the fourth electrode having afourth trace area substantially equal to the third trace area.
 21. Themethod of claim 16, wherein operating the electric pump in response tothe first field effect sensor detecting the presence of the fluidproximate to the first field effect sensor comprises operating theelectric pump in response to the first field effect sensor detecting thepresence of the fluid proximate to the first field effect sensor for apredetermined period of time.